Focuses on the potential substrate cycle between pyruvate, oxaloacetate (OAA), phosphoenolpyruvate (PEP), and pyruvate. Uses novel methods to test the following hypotheses: 1) the stimulation in net gluconeogenesis that occurs with fasting results from both a stimulation of phosphoenolpyruvate carboxykinase (PEPCK) and an inhibition of pyruvate kinase; and 2) gluconeogenesis in short term fasting is limited by fructose diphosphatase activity.